In some image processing devices such as a digital copying machine and a scanner, image processing is carried out so as to improve an image quality with respect to image data which have been read out by a photoelectric conversion device such as CCD. As such image processing, an enhancement process for enhancing a border region where image density is changed, and a smoothing process for suppressing an abrupt change in image density are available. However, in order to improve the image quality, it is required to select and carry out an image process which is most suitable for the feature of the image.
As such image processing, Japanese Unexamined Patent Publication No. 188948/1992 (Tokukaihei 4-188948) discloses an arrangement wherein judgement of a region is carried out per feature of an image in accordance with an average variance which has been determined from a difference between an average value of image data and each image data in a specific region.
Also, Japanese Unexamined Patent Publication No. 270561/1992 (Tokukaihei 4-270561) discloses a method for discriminating between a character image region and a halftone region in accordance with two threshold values THb (larger) and THc (smaller) that are set beforehand. More specifically, a region whose density level exceeds the threshold value THb and a region whose density level is lower than the threshold value THc are judged as a character image region, while a region whose density level falls in a range between the threshold value THb and the threshold value THc is judged as a halftone region.
Further, U.S. Pat. No. 4,722,008 discloses a method in which an image is divided into a plurality of blocks, each composed of pixels, and a change in signal levels of two succeeding pixels in each block is measured with respect to a main scanning direction and a sub scanning direction so as to discriminate whether each block is a spot region in accordance with the result of comparison between the sum of measured value of each block and a preset value.
However, the conventional image discriminating methods have a problem that respective features of image types of a character image, a spot image, and a photographic image cannot be extracted accurately, and the image region cannot be discriminated with high accuracy.
Namely, it is not necessarily the case that the average variance in the specific region is clearly changed in accordance with the image type, and it is often the case that the image types cannot be discriminated solely by the average variance. Also, in the method in which the density level is compared with the threshold values, it is difficult to set the threshold values for distinguishing image types, and in the case where the threshold values are set inappropriately, the discrimination accuracy of image types is greatly lowered. Further, in the method as disclosed in U.S. Pat. No. 4,722,008, while it is possible to discriminate whether the image region subjected to discrimination is a binary image region or a halftone region, it is impossible to accurately distinguish a character image from a photographic image.